Potassium niobate (KNbO3) is a well-known electro-optic material, which when doped with impurities that act as donors and acceptors the crystal becomes photorefractive. When forming an interference pattern inside the crystal with two mutually coherent laser beams, the donor ions are photoionized and the delocalized charges are trapped by the acceptor ions in the dark regions. Through the linear Pockel's effect, the periodic space-charge field generates a periodic index of refraction, i.e., photorefractive grating. In the case where a phase shift between the periodic index of refraction and the interference pattern exists, the power from one beam couples into the other beam, this effect maximizes with a π/2 phase shift.
A key problem associated with doped KNbO3 crystals is the lack of uniform photorefractive properties (i.e. non-uniform space-charge field). Several Fe-doped KNbO3 crystals are available from various international growers. It has been repeatedly observed that small regions throughout the crystals, particularly near the edges, exhibit strong counter-propagating two-beam coupling (TBC) efficiencies. These regions of enhanced coupling exhibit counter-propagating TBC efficiencies, where the TBC efficiency is defined as the ratio of the instantaneous transmitted intensity to the steady-state transmitted intensity, of nearly two orders of magnitude greater than the majority of the crystal as measured at 532 nm; 1/e response times are typically several hundred microseconds when the grating is recorded using powers of a few milliwatts at f/20.